05 Identifying Pure Substances

Unknown liquid challenge

Preliminary activity for

Identifying a Pure Substance

Open Inquiry Version

Physical properties of a substance are characteristics that can be observed without altering the identity of the substance. Color, odor, density, melting temperature, boiling temperature, and solubility are examples of physical properties. Physical properties can be used to identify a pure substance.

The Table 1 density values were determined at 20ºC and the boiling temperatures were determined with pressure equal to 101.3 kPa.

vs = very soluble, ss = slightly soluble, and nos = not observably soluble.

In the Preliminary Activity, you will gain experience using a Temperature Probe and data-collection software as you determine the boiling temperature of 2-propanol.

After completing the Preliminary Activity, you will first use reference sources to find out more about physical properties of pure substances and how to determine physical properties. You will devise a plan (see the back of this packet) for identifying an unknown from among substances listed in Table 1, and then carry out your plan to identify the unknown. Some topics to consider in your reference search are:

Unknown liquid challenge5 - 1 S

Unknown liquid challenge

• substances
• physical properties
• characteristic physical properties
• density
• melting temperature
• boiling temperature
• solubility
• chemical properties

Unknown liquid challenge5 - 1 S

Unknown liquid challenge

Figure 1

Procedure

1.CAUTION: Handle hot water and hot equipment with care throughout this experiment. There should be no open flames in the laboratory during this experiment. Never smell any chemical directly. When testing for odor, use a wafting motion to direct odors to your nose.

2.Obtain and wear goggles.

3.Connect the Temperature Probe to the data-collection interface. Set the data-collection program up to collect data for 900 seconds following your instructor’s directions.

4.Prepare a 2-propanol sample for data collection.

1. Measure about 8 mL of 2-propanol into an 18  150 mm test tube.
2. Add 3 or 4 boiling beads to the test tube.
3. Arrange a hot plate on or next to the base of a ring stand as shown in Figure 1.
4. Fill a 400 mL beaker 2/3 full with hot tap water and place the beaker on the hot plate.
5. Use a utility clamp to attach the test tube containing the 2-propanol to the ring stand. Lower the test tube into the hot tap water until it is about 1 cm from the bottom of the 400 mL beaker. The water level in the beaker should be about 2 cm above the 2-propanol level in the test tube.
6. Fasten the Temperature Probe in a second utility clamp. Lower the Temperature Probe into the test tube until the tip of the probe is about 0.5 cm above the surface of the
   2-propanol. Fasten the utility clamp to the ring stand with the Temperature Probe in this position.
7. Turn the hot plate to the temperature setting suggested by your instructor.

5.Collect data.

1. Begin data collection.
2. When the water comes to a rolling boil, turn the hot plate setting down to a setting just high enough to maintain boiling.
3. When the 2-propanol has boiled with noticeable bubbling for 100 s or until about 3/4 of it has boiled away, which ever happens first, stop data collection.
4. Carefully raise the test tube above the water.
5. Allow the beaker, water, and hot plate to cool before handling them.

6.Determine the boiling temperature of 2-propanol:

1. Select the flat part (plateau) of your curve.
2. Use the Statistics function to determine the mean temperature during this plateau.
3. Record this temperature as the boiling temperature of 2-propanol.

Questions

1.Which value did you obtain for the boiling temperature of 2-propanol in the Preliminary Activity? How does your value compare to the value given in Table 1?

2.List two factors that can affect the boiling temperature of a pure substance. How do these factors affect boiling temperature?

3.What is the normal maximum temperature of a hot-water bath like the one used in this experiment?

4.What can be done to increase the maximum temperature of a hot-water bath?

Note: The plan (on the back of this page) that you submit for instructor approval should list laboratory safety concerns, including chemical safety concerns, and specify how you will address these safety concerns during your investigation..

Researchable Question

Variables

Materials List

Safety Issues

Procedure

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Data Table

Conclusion

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